This invention relates to electric power supplies, and particularly to a switching power supply capable of a.c.-to-d.c. voltage conversion, featuring provisions for attainment of closer approximation of the input current waveform to a sinusoidal wave, and for that of a higher power factor, than by the comparable prior art.
The switching power supply or voltage regulator has long been familiar which comprises a rectifying and smoothing circuit to be coupled to a source of a.c. power, and a d.c.-to-d.c. converter circuit connected to the rectifying and smoothing circuit. The rectifying and smoothing circuit comprises a rectifier circuit and a smoothing capacitor. Although so simple in configuration, this known rectifying and smoothing circuit possesses the disadvantage of a somewhat poor power factor as a result of the fact that the smoothing capacitor is charged only at or adjacent the peaks of the a.c. voltage of sinusoidal waveform. Another drawback is that the input current is not favorable in waveform.
Designed to defeat these shortcomings, a more advanced switching power supply has also been suggested which comprises an inductor connected between the rectifier circuit and the smoothing capacitor, and a switch which is connected between the pair of outputs of the rectifier circuit and which is controllable via the inductor. The smoothing capacitor is connected in parallel with the switch via the rectifying diode. This known circuit comprising the inductor and the switch is sometimes referred to as the step-up power-factor improvement circuit. As the switch is turned on and off at a repetition frequency higher than the frequency of the input a.c. voltage, the current flowing through the inductor has a peak value in proportion with the instantaneous value of the input voltage. The results are a close approximation of the input current waveform to a sinusoidal waveform, and an improvement in power factor. It is also possible to make the voltage across the smoothing capacitor higher than the maximum value of the a.c. voltage.
Offsetting these advantages of the prior art device is the fact that switching losses occur both at the switch included in the step-up power-factor improvement circuit and at that in the d.c.-to-d.c. converter circuit connected thereto, resulting in a substantial decrease in efficiency. It must also be taken into consideration that the on-off control of the power-factor improvement circuit switch and d.c.-to-d.c. converter circuit switch at different repetition frequencies is undesirable. Noise was indeed easy to be produced according to this switch-driving scheme, and the switches prone to become unstable in operation, as a result of the mutual interference of the different driving frequencies of both switches.
Japanese Unexamined Patent Publication No. 8-154379 suggests a different type of switching power supply. The switch in the d.c.-to-d.c. converter circuit is utilized for switching both the d.c. voltage across the smoothing capacitor and the current through the inductor for power factor improvement. One switch performs the dual purpose of power factor improvement and d.c.-to-d.c. conversion, but to lesser extents than by two switches.
The present invention has it among its objects, in a switching power supply of the type defined, to further improve the power factor and, at the same time, to most effectively and inexpensively lessen power losses due to the switch included in the power-factor improvement circuit and that in the d.c.-to-d.c. converter.
Briefly, the present invention may be summarized as a switching power supply capable of translating a.c. voltage of sinusoidal waveform into d.c. voltage. Included is a rectifier circuit connected to a pair of input terminals for rectifying the input a.c. voltage, the rectifier circuit having a first and a second output for providing a rectifier output voltage. A first main switch is connected to the first output of the rectifier circuit via a main inductor on one hand and, on the other hand, to the second output of the rectifier circuit. The first main switch has capacitance means for its soft switching, the capacitance means being in the form of either a discrete capacitor connected in parallel therewith or its parasitic capacitance. A rectifying diode is connected to the rectifier circuit via the main inductor. A smoothing capacitor is connected in parallel with the main switch via the rectifying diode. A transformer is provided which has a primary winding through which a second main switch is connected in parallel with the smoothing capacitor. The second main switch has its own soft-switching capacitance means. A rectifying and smoothing circuit is connected to the transformer for providing output d.c. voltage. A first ancillary inductor is connected to the main inductor and electromagnetically coupled thereto. An ancillary switch is connected to the main inductor via the first ancillary inductor on one hand and, on the other hand, to the second output of the rectifier circuit. A first reverse-blocking diode is connected in series with the first ancillary inductor. Electromagnetically coupled to the primary winding of the transformer, a second ancillary inductor has one extremity connected to a junction between the second main switch and the smoothing capacitor, and another extremity connected to the ancillary switch. A second reverse-blocking diode is connected in series with the second ancillary inductor. Also included is a switch control circuit which is connected to the first main switch for on-off control thereof at a repetition frequency higher than the frequency of the input a.c. voltage, to the second main switch for on-off control thereof so as to cause d.c. voltage to be intermittently applied from the smoothing capacitor to the primary winding of the transformer, and to the ancillary switch for on-off control thereof at such a repetition frequency, and with such conducting periods, as to assure soft turn-on of the first and the second main switch.
The invention as summarized above offers the following advantages:
1. The first soft-switching capacitor and the first ancillary inductor constitute in combination a resonant circuit conducive to the soft switching of the first main switch. As a result, less power loss and less noise occur at the first main switch, and the power factor is improved with the power loss kept at a minimum.
2. The second soft-switching capacitance means and the second ancillary inductor constitute in combination a second resonant circuit conducive to the soft switching of the second main switch. This second main switch is therefore also kept from power loss and noise production, and d.c.-to-d.c. conversion is accomplished with minimal power loss.
3. The first and the second resonant circuit share the ancillary switch for soft-switching the first and the second main switch, rather than employing separate ancillary switches for the same purposes.
4. The electromagnetic coupling of the first ancillary inductor with the main inductor is effective to restrict the current flowing through the main inductor into the smoothing capacitor when the ancillary switch is turned on with consequent voltage application to the first ancillary inductor. The current thus flowing into the smoothing capacitor will drop to zero in a relatively short period of time, resulting in early commencement of discharge by the first soft-switching capacitance means.
5. Similarly, as a result of the electromagnetic coupling of the second ancillary inductor with the transformer primary, the energy release from transformer primary to rectifying and smoothing circuit is completed in a relatively short period of time when the ancillary switch is turned on with consequent voltage application to the second ancillary inductor. The result is an early commencement of discharge by the second soft-switching capacitance means.
6. Electromagnetically coupled together, moreover, the main inductor and the first ancillary inductor are manufacturable as a compact, integral combinations.
7. The transformer primary and the second ancillary inductor are likewise electromagnetically coupled together. The transformer is manufacturable in compact, integral combination with the second ancillary inductor.
It is also an advantage of this invention that the two main switches are both driven at the same switching frequency. The switch control circuit is much simpler and inexpensive in construction than if the main switches are driven at different frequencies. The driving of the main switches at different frequencies is objectionable for an additional reason: The different driving frequencies would be difficult of creation because of their possible mutual interference. Noise production would also be easier to occur.
In some embodiments of the invention, not only are the two main switches driven at the same frequency, but they are turned on simultaneously. The switch control circuit can then be further simplified in construction.
In some other embodiments, however, the two main switches are turned on at different moments. One such embodiment employs what are termed conducting period limitation signals for variously delaying the beginnings of predefined tentative conducting periods of the main switches. The conducting period limitation signals permit the main switches to be turned on at different moments that are independently adjustable.
A yet further embodiment is disclosed in which the main switches are turned on when the voltages across them drop below predetermined reference voltages. The main switches can then be turned on at zero or very low voltages more positively than in cases where they are turned on at moments predetermined by timers.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following detailed description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.